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How Many Spark Plugs Does A Diesel Have? (+4 FAQs)

March 12, 2022

If you’re a diesel engine owner, you may have wondered — how many spark plugs does a diesel have

You may have also heard of “glow plugs” in relation to that question.
What are glow plugs?

We’ll be tackling these questions in the following paragraphs. We’ll also cover what types of plugs a diesel engine uses and answer some common questions regarding diesel engine plugs. 

This Article Contains 

Let’s get started. 

How Many Spark Plugs Does A Diesel Have

The simple answer is — none
Unlike a gasoline engine (petrol engine), a diesel engine doesn’t use any spark plugs

But, then what burns the fuel inside the combustion chamber of a diesel engine? 
To answer this, we need to understand how a diesel and gasoline engine works. 

A gasoline or petrol engine uses an ignition coil to transform the battery’s voltage to a higher voltage. From the ignition coil, the high voltage is transferred to a distributor and then to each spark plug via the spark plug wires. 

The spark plug creates a tiny electric spark in the spark gap, igniting the air fuel mixture inside the combustion chamber.

However, diesel fuel doesn’t ignite by a spark as its combustion process is slightly different. 

Diesel fuel requires air compression to raise the temperature inside the combustion chamber. Under high temperatures, diesel combusts immediately upon contact with the compressed air supplied at the end of the compression stroke. 

So, instead of a spark plug, a compression ignition engine (like the diesel engine) uses a heating device called a glow plug.

But, what exactly is a glow plug?
Let’s find out. 

What Is A Glow Plug

A glow plug is similar to a spark plug in the sense that it’s a part of the ignition system of your car’s engine. 

However, instead of creating a spark, diesel engines use glow plugs to warm up the combustion chamber and aid ignition, especially in cold weather. 

A glow plug also keeps the cylinder head and cylinder block warm enough for a quicker engine start. 

How does this work?

When you turn on the ignition of your diesel vehicle, the air inside the internal combustion engine undergoes high compression. The resulting pressure forces the fuel injector to release diesel inside the chamber, forming an air fuel mixture. 

Meanwhile, the glow plug heats up to warm up the combustion chamber. When the chamber is warm enough and under high pressure, the airfuel mixture ignites, cranking the engine. 

This type of combustion may seem to take a long time, but a glow plug in modern diesel engines can reach a heat range of up to 1,000℃ in less than 2 seconds. 

Glow plugs operate in three phases: 

Most diesel engines run on one glow plug per engine cylinder. This means a four cylinder engine will have four glow plugs, a six cylinder diesel engine will have six plugs, and a V8 engine will have eight glow plugs. 

Now, when it comes to selecting a glow plug for your vehicle, you do have a couple of options. 

What Are The Different Types Of Glow Plugs?

Here are the three types of glow plugs available in the market: 

1. Steel Glow Plugs

Steel glow plugs are the standard and the cheapest type available. Because they’re made of steel, they can take longer to heat up the fuel and the internal combustion engine in colder climates. 

But if you don’t drive your diesel vehicle too often and experience moderate temperatures in your area, a steel plug is a good option.

2. Ceramic Glow Plugs

Ceramic glow plugs are more expensive than steel plugs. The ceramic material allows them to warm up faster and generate consistent heat. 

Ceramic glow plugs can also reach higher temperatures (up to 1,300°C), making them ideal for colder climates. 

3. Pressure Sensor Glow Plugs

Pressure sensor glow plugs are the most expensive type available. They help warm up the engine and give you accurate readings about compression levels inside the engine cylinder. 

When the air pressure inside the cylinder is low, they signal the vehicle’s ECU to adjust the compressed air levels or alert the driver through warning lights. This lets an owner know how efficiently their diesel engine is burning the fuelair mixture.

So even if they’re on the pricier side, these plugs are worth investing in to manage your vehicle’s fuel efficiency, fuel economy, and exhaust emissions.

Let’s go over some questions on spark plugs and glow plugs next. 

4 FAQs About Spark Plugs And Glow Plugs

Here are answers to some questions that you, as a diesel engine owner, might have: 

1. How Is A Spark Plug Different From Glow Plug? 

A spark plug is a critical part of a gasoline or gas engine. It generates a spark through the spark plug gap to ignite the air fuel mixture inside the combustion chamber. 

On the other hand, a glow plug heats up the compression ignition system so that combustion can take place easily. 

2. Can My Diesel Engine Work Without A Glow Plug?

While a spark plug is a necessary component of a petrol engine’s ignition system, a diesel engine doesn’t need a glow plug

So yes, a diesel engine can function even with a failing glow plug. But your petrol car won’t start or can have an engine misfire if you have a bad spark plug.

However, this only stands true for diesel vehicles that operate in hot climates. 

When it’s already warm enough outside, diesel fuel will flow easily to the fuel injector. As such, the fuel will ignite quickly upon contact with the heated compressed air inside the combustion unit. 

3. How Long Do Glow Plugs Last?

Your mechanic may suggest inspecting your glow plugs every 12,000 miles as a part of regular vehicle maintenance. 

A glow plug can typically last for 100,000 miles before it goes bad, and you should ideally replace all your glow plugs once you hit this mark. 

It’s also best to replace all the glow plugs at once, as one bad glow plug can affect your engine’s performance and fuel efficiency. 

4. How Will I Know If I Have A Failing Glow Plug? 

Since glow plugs generate incredibly high heat, they’re prone to wear out over time. 
Here are a few symptoms of a bad glow plug you should watch out for: 

Final Thoughts 

Now you can answer this question, “how many spark plugs does a diesel have?”

While modern diesel engines have no spark plugs, they do require glow plugs equal to the number of cylinders they have.

And if you need help replacing a faulty glow plug in your diesel car, contact RepairSmith!

RepairSmith is a convenient mobile car repair and maintenance solution offering competitive, upfront pricing.

Our ASE-certified mechanics can easily install a glow plug in your engine right in your driveway and take care of all your automotive maintenance needs.

Fill out this form for an accurate cost estimate for glow plug replacement or any other engine repair!